Introduce subfolders / restructure modules thematically

[mmacata]

and rename classes, methods + modules if more meaningfull name is possible.

[mmacata]

The folder structure to start with was agreed upon:

.
├── rest
│   ├── topic1
│   └── topic2
├── core
│   ├── common
│   ├── topic1
│   └── topic3
└── models
    ├── topic1
    └── topic2

while topic relates to colored boxes of this UML draft

[mmacata]

Status is: Code was sorted into rest, core and models folders

• modules from actinia_core.resources.common changed to actinia_core.core
  • but modules which are used by actinia plugins changed to actinia_core.core.common
• modules from actinia_core.resources changed to actinia_core.rest
• response_models moved to actinia_core.models.response_models
• models were moved out of some modules into separate model modules into actinia_core.models.openapi